U.S. patent application number 11/443005 was filed with the patent office on 2006-12-07 for semiconductor device.
This patent application is currently assigned to SANYO ELECTRIC CO., LTD.. Invention is credited to Akihiro Hasegawa.
Application Number | 20060273358 11/443005 |
Document ID | / |
Family ID | 37484364 |
Filed Date | 2006-12-07 |
United States Patent
Application |
20060273358 |
Kind Code |
A1 |
Hasegawa; Akihiro |
December 7, 2006 |
Semiconductor device
Abstract
In a laser pickup photodetector of an optical disk playback
device, the sensitivity to blue light is improved. On a main
surface of a semiconductor substrate, a high resistivity epitaxial
layer that becomes an i layer of a PIN photodiode (PIN-PD) is
formed. On a surface of the epitaxial layer, two trenches are
formed, on a surface of one trench an N.sup.+ region that becomes a
cathode region of the PIN-PD is formed, and on a surface of the
other trench a P.sup.+ region that becomes an anode region is
formed. When the cathode region and the anode region are set in a
reverse bias state, a light receiving semiconductor region that is
an i layer between the cathode region and anode region is depleted.
The depleted layer expands to a surface of the semiconductor
substrate. Accordingly, for blue light having a short wavelength,
signal charges can be generated on a surface of the semiconductor
substrate and the cathode region can collect the signal charges and
extract the charges as a light receiving signal.
Inventors: |
Hasegawa; Akihiro;
(Hashima-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
SANYO ELECTRIC CO., LTD.
Moriguchi-shi
JP
|
Family ID: |
37484364 |
Appl. No.: |
11/443005 |
Filed: |
May 31, 2006 |
Current U.S.
Class: |
257/290 ;
257/E31.039; 257/E31.061; 257/E31.115 |
Current CPC
Class: |
H01L 31/02024 20130101;
H01L 31/105 20130101; Y02E 10/50 20130101; H01L 31/03529
20130101 |
Class at
Publication: |
257/290 |
International
Class: |
H01L 31/113 20060101
H01L031/113 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2005 |
JP |
2005-164741 |
Claims
1. A semiconductor device comprising: a light receiving
semiconductor region that is disposed on a main surface of a
semiconductor substrate, receives signal light and has a low
impurity concentration; and an anode region and a cathode region
that are formed on the main surface with the light receiving
semiconductor region disposed therebetween, wherein the anode
region is a first conductivity type semiconductor region to which a
first voltage is applied and that has an impurity concentration
higher than the light receiving semiconductor region; the cathode
region is a second conductivity type semiconductor region to which
a second voltage is applied and that has an impurity concentration
higher than the light receiving semiconductor region; and, the
anode region and the cathode region are put into a reverse bias
state owing to the first voltage and the second voltage to form a
depletion layer in the light receiving semiconductor region.
2. A semiconductor device, having a light receiving portion divided
into a plurality of segments on a main surface of a semiconductor
substrate, comprising: a light receiving semiconductor region that
is formed on the main surface and has a low impurity concentration;
a plurality of first electrode regions formed on the main surface
for each of the segments; and a second electrode region formed on
the main surface along a boundary between the segments, wherein the
first electrode region is a first conductivity type semiconductor
region to which a first voltage is applied and that has an impurity
concentration higher than the light receiving semiconductor region;
the second electrode region is a second conductivity type
semiconductor region to which a second voltage is applied and that
has an impurity concentration higher than the light receiving
semiconductor region; and, the first electrode region and the
second electrode region are put into a reverse bias state owing to
the first voltage and the second voltage to form a depletion layer
in the light receiving semiconductor region therebetween.
3. The semiconductor device according to claim 2, wherein the first
electrode region for each of the segments is formed along a segment
boundary that does not face the other segments.
4. The semiconductor device according to claim 2, wherein the first
electrode region is a cathode region and the second electrode
region is an anode region.
5. The semiconductor device according to claim 1, wherein the anode
region or the cathode region is formed on a surface of a groove
portion formed on the main surface.
6. The semiconductor device according to claim 2, wherein the first
electrode region or the second electrode region is formed on a
surface of a groove portion formed on the main surface.
7. The semiconductor device according to claim 1, wherein the light
receiving semiconductor region is an epitaxial growth layer.
8. The semiconductor device according to claim 2, wherein the light
receiving semiconductor region is an epitaxial growth layer.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a semiconductor device that detects
light with a light receiving portion made of a PIN (p-intrinsic-n)
photodiode.
BACKGROUND OF THE INVENTION
[0002] In recent years, as an information recording medium, optical
disks such as CDs (Compact Disks) and DVDs (Digital Versatile
Disks) have become predominant. In a reproducing device of the
optical disks, an optical pickup unit irradiates laser light along
a track of an optical disk and detects reflected light thereof.
Based on a variation in intensity of the reflected light, recorded
data are reproduced.
[0003] An optical disk reproducing device, while detecting data
based on the reflected light, servo-controls a positional
relationship between the optical pickup unit and the optical disk.
Specifically, a tracking servo that allows irradiation of the laser
light along a centerline of the track and a focusing servo that can
keep a distance between the optical disk and the optical pickup
unit constant are implemented. For instance, in the focus servo
control, based on an output signal of the photodetector that
detects reflected laser light, an actuator variably controls a
position of the optical pickup unit to maintain a distance d with
the optical disk constant. As a result, an amount of reflected
light corresponding to a displacement of a focus of irradiation
light on a surface of the optical disk can be prevented from
fluctuating and thereby noise superposed on the light receiving
signal can be suppressed.
[0004] In order to obtain information for such servo control, as a
photodetector, a semiconductor device where a reflected light image
is divided into a plurality of segments and received is used. FIGS.
1 through 3 are schematic diagrams showing a light receiving
portion of the photodetector and a reflected light image on the
light receiving portion. Reflected laser light is input on the
photodetector through a cylindrical lens. The reflected light has a
circular section when it enters the cylindrical lens. According to
a principle of an astigmatism method, an image of the reflected
light after going through the cylindrical lens, in accordance with
a distance d between the optical pickup unit and the optical disk,
varies in a dimensional ratio in two perpendicular directions.
Specifically, when a distance d is a target value, as shown in FIG.
2, an image of reflected light is set so as to be a perfect circle
30. On the other hand, for instance, when the distance d is
excessive, as shown in FIG. 1, an image of reflected light becomes
a vertically long ellipse 32 and, when the distance d is
insufficient as shown in FIG. 3, an image of reflected light
becomes a horizontally long ellipse 34.
[0005] The photodetector has a light receiving portion that is
divided into 2.times.2=4 segments 36 and each of the segments
constitutes a light receiving element that outputs a light
receiving signal. The photodetector is arranged so that diagonal
directions of a 2.times.2 square arrangement of the light receiving
elements, respectively, may coincide with axes of the vertically
long ellipse 32 and the horizontally long ellipse 34. When the
light receiving elements are thus arranged, in FIGS. 1 through 3,
based on a difference between a sum of output signals of two light
receiving elements arranged on a diagonal line along a vertical
direction and a sum of output signals of two light receiving
elements arranged on a diagonal line along a horizontal direction,
shapes of the reflected light image as shown in FIGS. 1 through 3
can be distinguished. The shape of the reflected light image can be
used to control the distance d. On the other hand, the intensity of
light reflected in accordance with data can be obtained from a
total sum of output signals of four light receiving elements.
[0006] Since a data rate read from an optical disk is very high,
the photodetector is constituted of a semiconductor device that
uses a PIN photodiode having high response speed. FIG. 4 is a
schematic sectional view of an existing photodetector. The drawing
expresses a sectional view that goes through two adjacent light
receiving elements and is vertical to a semiconductor substrate.
The semiconductor device has a P.sup.+ region that becomes an anode
region 42, which is formed on a surface of a P-type semiconductor
substrate 40. Above the anode region 42, an i layer 44 that has a
low impurity concentration and high resistivity is formed by an
epitaxial growth method. In the i layer 44, at a position
corresponding to a boundary of the light receiving elements, an
isolation region 46 that is made of a P.sup.+ region and continues
to the anode region 42 is formed. Furthermore, on a surface of the
i layer 44, an N.sup.+ region that becomes a cathode region 48 is
formed.
[0007] The anode region 42, the i layer 44 and the cathode region
48 constitute a PIN photodiode that becomes a light receiving
element of a photodetector. The anode region 42 and the cathode
region 48, respectively, are connected to voltage terminals and a
reverse bias voltage is applied therebetween. In a reverse bias
state, in the i layer 44 between the anode region 42 and the
cathode region 48, a depletion layer is formed and electrons
generated in the depletion layer owing to absorption of incident
light move to the cathode region 48 owing to an electric field in
the depletion layer, followed by outputting as a receiving light
signal. Here, the isolation region 46, as mentioned above, reaches
the anode region 42 from a surface of the i layer 44. As a result,
the i layer 44 is divided for every light receiving element thus
making it possible to inhibit crosstalk between light receiving
elements.
[0008] A thickness of the i layer 44 is set equal to or more than a
substantial absorption length of detecting light in a
semiconductor. For instance, an absorption length of silicon to
light of a 780 nm or 650 nm band that is used in, for instance, a
CD or DVD is substantially 10 to 20 .mu.m. The P.sup.+ layer of the
isolation region 46 is formed, after the ion implantation, by
pressing in a depth direction by means of thermal diffusion.
However, at that time, in the thermal diffusion, the P.sup.+ region
is expanded not only in a depth direction but also in a horizontal
direction. In this connection, when the i layer 44 is relatively
thick, in order to form an isolation region 46 that is restricted
in width, the i layer 44 is formed divided into a plurality of
times of epitaxial growth. In this case, every time an epitaxial
layer 50 is formed, the ion injection and thermal diffusion are
carried out from a surface thereof and thereby an isolation region
52 reaching a bottom surface of the epitaxial layer 50 is formed.
When the epitaxial layers 50 and isolation layers 52 are thus
layered, the isolation region 46 extending in a depth direction can
be formed with a width that is prevented from expanding.
[0009] In a semiconductor device that constitutes an existing
photodetector, a cathode region 48 is disposed on a surface of a
semiconductor substrate, an i layer 44 located below the cathode
region 48 forms a depletion layer, and in the depletion layer
signal charges are generated by photoelectric conversion. In this
configuration, there is a problem in that it is difficult to detect
light of an absorption length that is substantially a thickness of
the cathode region 48 or less, namely, relatively short in
wavelength, for instance, blue light, being absorbed in the cathode
region 48. The problem becomes particularly important when an
optical disk reproducing device compatible with short wavelength
light capable of improving the recording density is being
realized.
[0010] Furthermore, in a semiconductor device that constitutes an
existing photodetector, when a relatively thick i layer such as 10
to 20 .mu.m is formed, the formation of an epitaxial layer 50 and
an isolation layer 52 is repeated a plurality of times.
Accordingly, there is a problem in that a semiconductor device is
high in manufacturing cost. There is a further problem in that, for
a part of a junction area of the isolation region 46 and the i
layer 44, a capacitance between terminals of an anode and a cathode
increases and as a result the high speed responsiveness that is a
feature of the PIN photodiode is damaged.
[0011] The invention intends to provide a semiconductor device
capable of detecting a short wavelength light component and
reducing the manufacturing cost and having responsiveness suitable
as a photodetector that detects a light signal from an optical disk
and so on.
[0012] [Patent literature 1] JP-A-10-107243
[0013] [Patent literature 2] JP-A-2001-60713
SUMMARY OF THE INVENTION
[0014] A semiconductor device according to the invention includes a
light receiving semiconductor region that is disposed on a main
surface of a semiconductor substrate, receives signal light and has
a low impurity concentration; and an anode region and a cathode
region that are formed on the main surface with the light receiving
semiconductor region interposed therebetween, the anode region
being a first conductivity type semiconductor region to which a
first voltage is applied and that has an impurity concentration
higher than the light receiving semiconductor region, the cathode
region being a second conductivity type semiconductor region to
which a second voltage is applied and that has an impurity
concentration higher than the light receiving semiconductor region,
the anode region and the cathode region being put in a reverse bias
state owing to the first voltage and the second voltage to form a
depletion layer in the light receiving semiconductor region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic diagram showing a light receiving
portion of a photodetector and an image of reflected light when a
disk is remote from a target distance.
[0016] FIG. 2 is a schematic diagram showing a light receiving
portion of a photodetector and an image of reflected light on the
light receiving portion when a disk is at a target distance.
[0017] FIG. 3 is a schematic diagram showing a light receiving
portion of a photodetector and an image of reflected light when a
disk is nearer than a target distance.
[0018] FIG. 4 is a schematic vertical sectional view of an existing
photodetector.
[0019] FIG. 5 is a schematic plan view of a photodetector that is a
semiconductor device according to an embodiment.
[0020] FIG. 6 is a schematic vertical sectional view showing a
structure of a light receiving portion according to an
embodiment.
[0021] FIG. 7 is a schematic diagram showing a circuit
configuration when a photodetector is in operation and a potential
distribution in a vertical section.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] In the following, a mode for carrying out the invention
(hereinafter, referred to as embodiment) will be described with
reference to the drawings.
[0023] FIG. 5 is a schematic plan view of a photodetector that is a
semiconductor element according to the embodiment. A photodetector
60 is formed on a semiconductor substrate made of silicon. A
passivation film (not shown in the drawing) layered on a surface of
a semiconductor substrate is provided with an opening at a position
corresponding to a light receiving portion. The light receiving
portion receives light that goes through the opening and enters a
surface of the substrate divided into 2.times.2=4 segments 62.
[0024] Corresponding to each of the segments 62, a cathode region
64 (first electrode region) is formed on a surface of the
semiconductor substrate. Each of the cathode regions 64 is disposed
in an outer periphery of the light receiving portion. Furthermore,
on a surface of the semiconductor substrate between the respective
segments 62, an anode region 66 (second electrode region) is
disposed, and the anode region 66 isolates light receiving elements
for every segment 62.
[0025] The cathode region 64 is formed as an N.sup.+ region where
an N type impurity is diffused at a high concentration from a
surface of a trench 68 (first groove portion) having, for instance,
an L-shaped planar shape along an outer periphery of the light
receiving portion. On the other hand, the anode region 66 is formed
as a P.sup.+ region where a P type impurity is diffused at a high
concentration from a surface of a trench 70 (second groove portion)
having, for instance, a cross-shaped planar shape formed between
the segments 62. Each of the cathode regions 64 is connected
through a contact to a wiring (not shown in the drawing) formed of,
for instance, an aluminum (Al) layer, and functions as a cathode of
a PIN photodiode for every segment 62. On the other hand, the anode
region 66 is connected through a contact to wiring (not shown in
the drawing), and functions as a common anode to the respective PIN
photodiodes.
[0026] FIG. 6 is a schematic sectional view showing the structure
of a light receiving portion in a section that goes through a
straight line A-A' shown in FIG. 5 and is vertical to the
semiconductor substrate. The photodetector 60 is formed with a
semiconductor substrate where, on one main surface of a P-sub layer
80 that is, for instance, a P type silicon substrate, a
semiconductor layer that has a lower impurity concentration than
the P-sub layer 80 and has high resistivity is formed. The high
resistivity semiconductor layer layered on the P-sub layer 80 is
formed according to, for instance, an epitaxial growth method. The
epitaxial layer 82 constitutes an i layer of the PIN photodiode. A
low concentration impurity introduced in the epitaxial layer 82 is,
for instance, a P type impurity.
[0027] The cathode regions 64 and the anode region 66 are formed on
a surface of the epitaxial layer 82, the trenches 68 and 70. A
surface of the semiconductor substrate is etched to form the
trenches 68 and 70. After the trenches 68 and 70 are formed, a
photoresist is coated on a surface of the semiconductor substrate,
and the photoresist is patterned to form an opening surrounding the
trench 68. With the photoresist as a mask, an N type impurity is
ion implanted. When an implanting direction is tilted, the ion
implantation can be applied onto a sidewall of the trench 68 as
well, and as a result a cathode region 64 can be formed on a
surface of the trench 68, namely, a sidewall surface and a bottom
surface of the trench 68. Similarly, a mask having an opening
corresponding to the trench 70 is formed with photoresist, followed
by ion implanting a P type impurity, and as a result an anode
region 66 is formed on a surface of the trench 70, namely, a
sidewall surface and a bottom surface of the trench 70.
[0028] The formation processes of the cathode region 64 and anode
region 66 can include a thermal diffusion process applied after the
ion implantation process, as required. Furthermore, after the
cathode region 64 and anode region 66 are formed, an insulating
film is buried in the trenches 68 and 70 and as a result a surface
of a light receiving portion can be made flat.
[0029] As well as constituting a cathode and an anode of a PIN
photodiode, as mentioned above, the cathode region 64 and anode
region 66 that are formed with the trenches 68 and 70,
respectively, also have a function of surrounding each of the
segments 62 to isolate a PIN photodiode corresponding to each of
the segments 62 from an external semiconductor region.
Incidentally, such a configuration is known as an STI (Shallow
Trench Isolation) technology.
[0030] In an internal portion surrounded by the cathode region 64
and anode region 66 of each of the segments 62, an epitaxial layer
82 appears on a surface. As will be described below, the portion
becomes a semiconductor region (light receiving semiconductor
region 72) having the sensitivity to incident light to the light
receiving portion.
[0031] Operation of the photodetector 60 will now be described.
FIG. 7 is a schematic diagram showing a circuit configuration when
the photodetector 60 is in operation and a potential distribution
in an element section corresponding to FIG. 6. The cathode region
64 is put into a reverse bias state, with respect to the anode
region 66 and P-sub layer 80 that are set at a ground potential, by
means of a voltage source 90. Specifically, wiring from each of the
cathode regions 64 is connected to one input terminal of an
operational amplifier 92 and a positive voltage Vb from the voltage
source 90 is inputted to the other terminal of the operational
amplifier 92. In the operational amplifier 92, an output terminal
is connected through a resistance to the cathode region 64 to form
a current detector. With this configuration, Vb is applied to the
cathode region 64 and a voltage corresponding to the cathode
current can be extracted at an output terminal of the operational
amplifier 92.
[0032] In a section of the photodetector shown in FIG. 7, some
equi-potential lines are shown with dotted lines. The sectional
view shows that when a reverse bias voltage is applied between an
anode and a cathode of a PIN photodiode, a depletion layer expands
in an epitaxial layer 82 that constitutes an i layer. In the
photodetector 60, both of a cathode region 64 and an anode region
66 are disposed on a surface of a semiconductor substrate, and a
light receiving semiconductor region 72 located in the vicinity of
a surface of the semiconductor substrate constitutes an i layer
between the cathode region 64 and the anode region 66. With this
configuration, when a reverse bias voltage is applied, a depletion
layer expands in the vicinity of the surface of the semiconductor
substrate corresponding to the light receiving semiconductor region
72 as well.
[0033] A potential in the depleted layer becomes deeper from the
anode region 66 toward the cathode region 64. That is, a potential
well is formed at a position corresponding to each of the cathode
regions 64. Furthermore, a boundary portion between the segments 62
corresponding to a position of the anode region 66 becomes
shallower in potential to form a potential barrier to movement of
electrons and as a result element isolation of the PIN photodiode
can be realized for every segment 62.
[0034] Light incident on each of the segments 62 is absorbed in the
depletion layer and generates electron-hole pairs as signal charges
and electrons are collected with a nearby cathode region 64. An
amount of electrons collected by each of the cathode regions 64 is
detected through the operational amplifier 92 as a cathode current.
In the photodetector 60, signal charges are also generated by light
absorbed in the vicinity of the surface of the semiconductor
substrate region 72 and the signal charges can be detected from the
cathode region 64. As a result, signal charges generated by short
wavelength light absorbed in the vicinity of the surface of the
semiconductor substrate can be extracted as the light receiving
signal and it is possible to obtain sensitivity to short wavelength
light.
[0035] Incidentally, when the trenches 68 and 70 are etched using
anisotropic etching technology such as RIE (Reactive Ion Etching),
the trenches 68 and 70 can be formed slenderly, and as a result an
area ratio of a light receiving semiconductor region 72 to a
semiconductor substrate surface of each of the segments 62 can be
made larger. As a result, the sensitivity of a PIN photodiode in
each of the segments 62 can be improved.
[0036] Furthermore, since a junction area between the cathode
region 64 and anode region 66 and the epitaxial layer 82 can be
made smaller, a capacitance between terminals of a cathode and an
anode of the PIN photodiode can be kept smaller and excellent
responsiveness can be secured.
[0037] As the PIN photodiode of each of the segments 62, a
semiconductor device according to the invention includes a low
impurity concentration light receiving semiconductor region that is
disposed on a main surface of a semiconductor substrate and
receives signal light, and an anode region and a cathode region
formed on the main surface with the light receiving semiconductor
region disposed therebetween. The anode region is a first
conductivity type semiconductor region that has an impurity
concentration higher than the light receiving semiconductor region
and to which a first voltage is applied. The cathode region is a
second conductivity type semiconductor region that has an impurity
concentration higher than the light receiving semiconductor region,
and is supplied with a second voltage. The anode region and the
cathode region are set to a reverse bias state owing to the first
voltage and the second voltage to thus form a depletion layer in
the light receiving semiconductor region.
[0038] Furthermore, the photodetector is an embodiment of a
semiconductor device according to the invention, in which a light
receiving portion divided into a plurality of segments is formed on
a main surface of a semiconductor substrate. The semiconductor
device includes a light receiving semiconductor region having a low
impurity concentration and disposed on the main surface, a
plurality of first electrode regions disposed on the main surface
for each of the segments, and a second electrode region formed on
the main surface along a boundary between the segments. The first
electrode region is a first conductivity type semiconductor region
that has an impurity concentration higher than the light receiving
semiconductor region and to which a first voltage is applied. The
second electrode region is a second conductivity type semiconductor
region that has an impurity concentration higher than the light
receiving semiconductor region and to which a second voltage is
applied. The first electrode region and the second electrode region
are set to a reverse bias state owing to the first voltage and the
second voltage and as a result a depletion layer is formed in the
light receiving semiconductor region therebetween.
[0039] The first electrode region of each segment can be formed
along a segment boundary that does not face the other segments. In
the photodetector, the cathode region is the first electrode region
and the anode region is the second electrode region.
[0040] The anode region or the cathode region, as with the
photodetector, can be formed on a surface of a groove portion
formed on the main surface. Furthermore, the light receiving
semiconductor region, like in the photodetector, ca be formed with
an epitaxial growth layer.
[0041] According to the invention, a PIN photodiode can be
constituted with a semiconductor region on a surface of a first
groove portion and a semiconductor region on a surface of a second
groove portion as an anode and a cathode and with a light receiving
semiconductor region therebetween as an i layer. On a surface of
the semiconductor substrate interposed between the anode and
cathode, a region having a high impurity concentration is not
formed and, when the anode and cathode are set in a reverse bias
state, a region in the vicinity of a surface of the semiconductor
substrate is also depleted. As a result, the anode or cathode can
collect charges generated by light having a short wavelength
absorbed in the vicinity of the surface of the semiconductor
substrate and can extract as a light receiving signal, thus making
it possible to obtain sensitivity to the short wavelength
light.
[0042] Furthermore, according to the invention, in a configuration
where a light receiving portion is divided into a plurality of
segments, a second groove portion formed at a boundary between the
segments fulfills a function of isolating light receiving elements
for each of the segments. The second groove portion is formed after
a light receiving semiconductor region that becomes an i layer is
formed. That is, a process of layering a plurality of epitaxial
layers for forming an i layer and a process of forming an isolation
region in each of the epitaxial layers, which were described as an
existing technology, can be omitted. It is therefore possible to
prevent increase in manufacturing cost. The first and second groove
portions reach only a partial depth of a light receiving
semiconductor region from a surface thereof. Thereby, since a
capacitance between terminals of the anode region and the cathode
region can be suppressed low, a high-speed response can be
attained.
* * * * *